1. Field of the Invention
This invention relates to a method and apparatus for applying force to molten glass being formed into a continuous sheet of glass while floating on molten metal. More particularly the invention relates to a method and apparatus for more efficient regulation of forces to either spread the glass to thinner than equilibrium conditions or prevent the glass from becoming as thin as in equilibrium conditions.
2. Discussion of the Prior Art
It has been conventional practice in the float glass formation art to utilize knurled or toothed rollers arranged in pairs along the margins of the glass ribbon as it passes through the forming chamber to apply forces to the margins of the ribbon in order to form the ribbon into either greater or less than equilibrium thickness. The use of such rollers while resulting in satisfactory glass presents difficulties in that the rollers provide only point contact with the molten glass. Therefore, the molten glass is pulled inward between the roller members and then again is pulled outward as it passes each roller member. This creates unwanted distortions in the glass. Further, the series of edge control rolls on each side of the forming chamber require a large amount of coolant flow in order to cool the rollers and also there is a need for rotary seals in the water cooling system which are a source of difficulty. There is in addition, a need for a drive motor for each edge control roll and tachometers to allow reading of the speed of each roller in order to control its effect on the glass ribbon. Changes in the treatment of the glass by the rollers is difficult in that each roller must be individually adjusted as to angle and speed such that they may exert a combined effect in changing the equilibrium glass thickness of the ribbon. Therefore, there remains a need for improved edge control devices.
It has been proposed in U.S. Pat. No. 3,468,651 to Boaz that longitudinally arranged pipes having blades formed thereon be positioned along the edge portions of glass ribbon floating on the molten tin in the forming chamber. The blades would be stationary relative to the direction of the glass movement and could be adjusted to provide lateral stretching of the glass. Water cooled stationary blades are not particularly satisfactory as there is a tendency for the blades to stick to the glass as they are formed of metal. Further, the wear of the blades is high and the cooling provided by the blades to the glass creates additional distortional forces in the glass ribbon.
U.S. Pat. No. 3,563,720 to Ito et al discloses an edge control means comprising a refractory material having groove like cavities which is pressed down on the margins of a glass ribbon as it passes through the forming chamber. The refractory member of Ito may be porous and provided with means for discharging gases from the refractory control means against the glass. This device and method present problems in that the porous refractory is subject to plugging of the pores through which the gases are delivered. A further problem is that the tractive force available from the design having only grooves in the bottom portion is not sufficient in all cases to provide the grip needed to provide less than equilibrium thickness glass. The edge control device of Ito, et al is in contact with the tin which may lead to several kinds of difficulties including plugging of the pores of the refractory, contamination of the bath and decreases in the life of the edge control means.
Therefore, there remains a need for an edge control device that will not suffer from the disadvantages and low efficiency of the previous designs. The construction and maintenance of several banks of rollers requiring cooling and regulation means is expensive and limits the amount of other controls which may be allowed access to the furnace. Further, there is a need for edge control means which will decrease the amount of distortion which is caused in the glass by the uneven stretching of edge control rollers.